Components such as gas turbine blades, vanes and other cooled parts often contain cavities that distribute cooling air to a plurality of holes in the wall of the part that lead to the outer surface. Most turbine components are coated for protection from oxidation and/or corrosion with, for example, a metallic coating such as MCrAlY or a diffusion coating and some are also coated with a thermal barrier coating (TBC) for thermal insulation. The demands of operation of the parts in a gas turbine often lead to the degradation and consumption of the coating before the structural integrity of the underlying part itself is degraded. Hence, the metallic and ceramic coating must be removed and reapplied at least once during the lifetime of the component.
Thermochemical and physical processing of blades after initial manufacturing can be very problematic for parts with a large number of cooling holes. During original part manufacture, the coatings are usually applied first and then the holes are drilled directly through the coating and the wall thickness of the component. However, the holes are already in place during the repair operations, such as acid or alkaline chemical stripping, used to remove old coatings, or grit blasting, water-jet stripping, high speed grinding abrasive techniques and, high speed milling. As an example, U.S. Pat. No. 5,167,721 discloses a method of removing a plasma sprayed and sintered coating by liquid jet. A method of electrochemical stripping of turbine blades is disclosed in U.S. Pat. No. 6,165,345. Another method of removing an environmental coating on a metallic substrate is known from U.S. Pat. No. 5,851,409. The coating containing cracks is subjected to an acidic solution that penetrates the cracks and interacts with the diffusion zone so as to chemically strip the diffusion coating from the substrate. U.S. Pat. No. 6,174,448 discloses a method of removing a diffusion aluminide coating of a component. This method removes the coating by stripping aluminium from the coating without causing excessive attack, alloy depletion and gross thinning of the underlying superalloy substrate. Similar methods are known from U.S. Pat. No. 5,728,227 or EP-A2-861 919. Methods of removing a thermal barrier coating are known, e.g. from the documents U.S. Pat. No. 6,158,957, U.S. Pat. No. 6,132,520, U.S. Pat. No. 5,900,102, US-A1-2001/0009247, U.S. Pat. No. 6,210,488, U.S. Pat. No. 5,614,054.
Unfortunately, during processing the chemical agents that are applied to remove the coating also react to a certain extent with the base material or the recast zone of laser drilled cooling holes, resulting in dimensional changes—in particular widening of the holes. The change in shape or diameters of the holes brought about during processing can have a significant influence on the effectiveness of the cooling holes, especially considering that some holes are 1 mm or less in diameter. Specially shaped cooling holes are particularly susceptible to this as their effectiveness depends heavily on the accuracy of the shape of the hole. This problem is particularly great for the most modern components which contain hundreds of cooling holes and are designed to operate within very tight tolerance bandwidths—the upper limit on cooling hole diameter to stop the waste of unneeded cooling air which drastically reduces engine efficiency and power output and the lower limit on cooling hole diameter to prevent overheating of the component, which would lead to its premature failure in service. Chemical etching/stripping methods can lead to an attack of the surface layer of the cooling holes, leaving oversized holes that flow outside of the originally intended specification. Grit blasting and other physical processes involving grinding can erode away the edges and surface contour of the cooling holes at the upper surface of the component, again bringing the air flow to outside specified limits.
There have been several disclosures relating to this problem and there are several widely known practices. Those skilled in the art are aware that a common practice is to fill the holes with a suitable material such as plastic or wax. However the wear, thermal and chemical resistance of this material is often insufficient to maintain a desired level of filling in the cooling holes during preparation steps while processing which often involves surface grit blasting to clean the outer surface of the component. Due to its lack of wear resistance, the plastic or wax readily is eroded away by the grit blasting. Commonly used plastics are easily infiltrated by the strong acid solutions leading to their spallation and subsequent attack of the no longer protected base material.